Improvement: Add unit tests for drawing functions

[Cardosaum]

Currently, we lack unit tests for pretty much all drawing functions, so if we change the implementation for any of them, we don't have any automated method to assert that we still have a working version of such function.

Ideally, we would have unit tests for all the following drawing functions:

• [ ] Circle
• [ ] Circle Filled
• [ ] Rect
• [ ] Rect Filled
• [ ] Set Pixel
• [ ] Line
• [ ] Sprite
• [ ] Clear Screen